1. Field of the Invention
This invention relates to a transfer film for use in electrophotographic copiers.
2. Description of the Prior Art
Tracing paper has been widely used as a transfer paper for use in electrophotographic copiers. Such paper is not sufficiently high in dimensional stability, keeping quality, mechanical strength and water resistance, and if such properties are required, it is replaced by a transfer film formed from a matte film made of a polyethylene terephthalate (hereinafter referred to as PET resin) or cellulose triacetate resin (hereinafter referred to as CTA resin). However, to have the toner fixed adequately to the transfer film during transfer on an electrophotographic copier, the thermal fixing unit of the copier must be held at a high temperature, at least as high as 250.degree. C., or even higher than 300.degree. C. if the heating mechanism of the unit is of radiant-heating type. Accordingly, if the PET film or CTA film used as transfer film is not adequately resistant to high temperatures, it undergoes noticeable rippling (i.e., formation of waves on the surface) after thermal fixation, thus severely damaging the flatness of the surface, and shows a considerable degree of heat shrinkage. If the temperature at the thermal fixing unit is decreased to a level that does not cause rippling, the toner is fixed (i.e., adhered) to the film so weakly that it will easily separate from the film, thus making the film unsuitable for practical use. Further, even a heat resistant film of the aforementioned type does not provide satisfactory writing quality or adequate toner fixation. "Writing quality" refers to the ability of a transfer film to be written on with pencil, ink, or the like; this is an important property for a transfer film, in that it is often desirable to modify an image on a transfer film by ordinary writing thereon. In addition, the known types of transfer film are of such high transparency that it is difficult to detect them when stuck in the electrophotographic copier. Thus, the conventional method of using the interruption of light to detect a paper or film stuck in an electrophotographic copier fails to operate in the intended manner, and the copier will not stop even if the paper or film does become stuck.